Submarine reservoirs

ABSTRACT

Submarine reservoirs of large dimensions, of the type designed to be towed to the desired location for installation on the seabed, are immersed by drawing compensatory gas stored in the liquid state in an auxiliary enclosure associated with the reservoir, whereby the reservoir is submerged and when located the auxiliary enclosure can be detached from the installed submarine reservoir.

United States Patent [191 Bremaud [111 3,834,337 Sept. 10, 1974SUBMARINE RESERVOIRS [75] Inventor: Claude R. Bremaud, Merignac,

France [73] Assignee: Societe anonyme dite: Societe Europeenne DePropulsion, Puteaux, France [22] Filed: Feb. 22, 1973 [21] Appl. No.:334,630

[30] Foreign Application Priority Data Apr. 19, l972 France 72.13823[52] US. Cl. 114/16 E, 9/8 R, 114/52 7 [51] Int. Cl. B63g 8/22 [58]Field of Search 114/16 R, 16 E, 52, 68;

[56] References Cited v UNITED STATES PATENTS 3,340,842 9/1967 Winslowll4/68 3,376,588 4/1968 Berteaux et al 9/8 R Primary ExaminerTrygve M.Blix Assistant ExaminerGregory W. OConnor 5 7] ABSTRACT Submarinereservoirs of large dimensions, of the type designed to be towed to thedesired location for installation on the seabed, are immersed by drawingcompensatory gas stored in the liquid state in an auxiliary enclosureassociated with the reservoir, whereby the reservoir is submerged andwhen located the auxiliary enclosure can be detached from the installedsubmarine reservoir.

7 Claims, 3 Drawing Figures PAIENTEDSEP] 01924 SHEET 2 BF 2 FIG.2

FIG. 3

SUBMARINE RESERVOIRS The present invention relates to a method of immersing and placing on the seabed a concrete submarine reservoir of largedimensions designed to store a considerable quantity of hydrocarbonextract below water. The invention relates more particularly to a methodof regulating the pressure inside the reservoir during immersion, thereservoir containing a flotation gas, in which method an inertcompensatory gas is introduced into the reservoir until the pressurepresent inside it is at least approximately equal to that of thesurrounding medium in which the reservoir is immersed.

The invention relates equally to a concrete submarine reservoir designedto be immersed and located on the seabed.

Submarine reservoirs designed for location on the seabed are, as isknown, filled with a flotation gas, generally air, so that they canfloat on the water and can be pulled along from the shipyard where theyare constructed to the point where they are to be immersed. Theimmersion operation is made possible by making the reservoir almostnon-buoyant, for example by introducing a suitable quantity of seawaterinto the reservoir, and keeping the buoyancy of the reservoir virtuallyat zero during immersion. For this purpose, the enclosure forming thereservoir'is constructed so that it can be isolated from the exterior,for example by gates; in this way, by closing the isolation gates, it ispossible to keep constant the volume occupied by the flotation gasduring the whole operation of immersion of the reservoir. However,because of this isolation, the external walls of the reservoir aresubject during immersion to differential pressures which increaseprogressively as the reservoir sinks into the water, so that thereservoir is liable to be damaged or even destroyed well before reachingthe bottom of the water. It is therefore necessary to regulateconstantly the internal pressure of the reservoir during its descent, byintroducing an inert pressure-compensating gas into the enclosing wallof the reservoir during immersion, so that the pressure inside theenclosure is constant and at least approximately equal to the pressureof the external surrounding medium in which the reservoir is located.

According to a known method, compressed air is used as thepressure-compensating gas, the source of which is located on the surfaceof the water and is connected to the reservoir by a connecting pipe ofvariable length. Since the volume of compensatory gas necessary forimmersion is usually very large, the source of compressed air must be oflarge dimensions and therefore expensive; furthermore, the connectingpipe comprises special equipment which is therefore expensive, not onlybecause of the variable distance between the reservoir to be fed withcompressed air and the surface of the water, but also because of thegenerally increased maximum value of the pressure of the compressed airwhich is necessary to bring about regulation of the pressure mentioned,this increased pressure being withstood by the connecting pipe.

The invention seeks to remedy these disadvantages and provides, inparticular, an economical and safe method of regulating the internalpressure of the reser voir.

For a method of the type mentioned above, this is achieved in accordancewith the invention in that before it is introduced into the-reservoir,the compensatory gas is first stored in the liquid state in an auxiliaryenclosure or chamber associated with the reservoir.

In this way, the pressure-compensating gas is stored in compact form andit is no longer necessary to connect the concrete reservoir to a sourceof compensatory gas located at the surface of the water; the result ofthese advantages is that the immersion operation is greatly simplified.

According to an advantageous and preferred feature of the invention, thecompensating gas is introduced in the liquid state into an aqueoussolution, i.e. an auxiliary solution, which is contained in a chamberforming part of the reservoir and constantly in communication with thespace in the reservoir filled with the flotation gas. The auxiliarysolution can simply be, for example, the seawater which hasalready beenintroduced to give the reservoir virtually no buoyancy. This auxiliarysolution serves as a heat source and also as a thermal damping force orheat store, which can supply the heat necessary to convert intocompensatory gas the liquid introduced into the auxiliary solution. Thissolution advantageously comprises brine which can be prepared fromseawater which is introduced into the reservoir and'from the addition,for example, of a suitable quantity of sea salts. An auxiliary solutionis thus obtained which has a relatively low freezing point, which islower than that of ordinary seawater.

The compensatory gas advantageously comprises CO As is known, this gascan easily be liquefied and can be stored economically at lowtemperatures under pressure. Furthermore, CO is very soluble in water,seawater or brine. If CO is usedas compensatory gas, as is preferred,the auxiliary solution serving as the intermediate medium between thesource of liquid CO and the gas-receiving volume of the reservoirenclosure is permanently supersaturated with dissolved CO and liberatesgaseous CO into the gas-receiving space as soon as a quantity of liquidCO is introduced and dissolved in the auxiliary solution at a pointlocated below the maximum level of the solution. In this way, theformation of carbonic frost or snow is avoided, which can occur as isknown when carbon dioxide gas is released adiabatically.

The CO is preferably stored at a temperature of about 20C under apressure of about 20 bars.

The method according to the invention is advantageously carried out bymeans of a concrete reservoir which comprises at least one enclosurewhose volume is symmetrically or otherwise uniformly associated with theperiphery of the concrete body of the reservoir and is filled with anauxiliary aqueous solution, at least one second enclosure which isarranged in a central zone of the concrete body of the reservoir whichis filled with flotation gas and which is in constant communication withthe first enclosure and a chamber filled with liquid CO which isassociated with the concrete body of the reservoir and is connected tothe first enclosure by means of a valved supply pipw which enters thefirst enclosure at a point below the maximum level of the auxiliarysolution.

Such a reservoir produces a moment of inertia around its raised centre,since the ballast material which in the'present case comprises theauxiliary aqueous solution contained in the first enclosure isdistributed uniformly in relation to the periphery of the reservoir, sothat the latter is very stable as regards its inclination to thehorizontal and can thus be kept horizontal very easily during itsimmersion. Furthermore, in this new reservoir, the ballast liquid alsoserves as the auxiliarysolution receiving CO in a liquid state andintroducing CO in a gaseous state into the second enclosure.

Other characteristics and advantages of the invention will be seeninf'connection with the following description of an illustrativeembodiment of a submersible reservoir according to, the invention, thedescription being given with reference to the accompanying drawings.

FIG. 1 is'an axial section partially in elevation of an embodiment ofthe reservoir according to the invention.

FIG. 2 is vertical cross section of the apparatus FIG. 3 is crosssection taken along the line llll of FIG. 2 showingthe concentricarrangement of enclosures 2 and 3.

As can be seen from the drawing, the reservoir consists of a body ofconcrete which comprises a principal part in the form of a cylindricalplate 1, in which enclosures are formed which are filled with brine 2and which are uniformly distributed around the periphery of thereservoir and air-filled enclosures 3 which are located in the centralzone of the reservoir and which are in constant communication with theenclosures 2 by means of apertures 5 in the upper part of the divisionsseparating the enclosures 2 and 3 from one another. The concrete body ofthe reservoir further comprises a chimney 4 whose height is equal to orslightly higher than the depth of the under-water site on which thereservoir is to be placed.

On the upper part of the reservoir, a frame '7 is fixed by means ofexplodable bolts 6, which frame supports a caisson 8 containing the COstoring unit. Horizontally arranged in the caisson 8 is a cylindricaltank 9 filled with liquid CO at'a pressure of 20 bars and a temperatureof 20C. The tank 9 is of steel and is covered on its exterior with aninsulating coating 10 which can resist the surrounding pressure andwhich consists for example of a layer offoam polyurethane covered with alayerof polyester. The tank 9 is mounted on supported cradles mounted onthe frame 7.

In the tank 9 is located an evaporator 11 which is connected by means ofrigid pipes equipped with gate valves, which are not illustrated, to arefrigerator unit 12 installed inside the caisson 8. The refrigeratorunit 12 serves to keep the liquid contained in the tank 9 at thetemperature of C during the period between filling of the tank andimmersion of the reservoir. A double-inversion member, comprising athreeway electric valve 15, safety valves 16 and pressure-rupturablediscs 17, is similarly connected to the interior of the tank- 9; thesemembers 15, 16, 17 form a safety device which precludes an increase inpressure in the interior of the tank 9 caused by overheating of the COduring storage. Filling and balancing valves 13 and 14 for the tank 9are provided to allow liquid CO to be supplied to the tank 9 fromtankers which are not illustrated and which can be loaded on thereservoir when it is in the quayside.

The enclosures 2 of the concrete reservoir are each connected to. theinterior of the liquid CO tank 9 by means of supply pipes on which areplaced pneumatic valves 19 which are controlled byelectric gate valves18 and by explosive cutter devices 22.

The ends of the supply pipes each connected to an enclosure 2 areequipped with injection nozzles 20 which are entirely submerged in thebrine solution. Furthermore, the tank 9 is equipped with a'level tester23 and a pressure senser 24.

An electric junction box 25 is similarly provided in 9. In these areinstalled the valves 13 to 19, the refrigcrating unit 12 and theelectric equipment 25. A constant flow of CO gas through the nozzle 29and an external purge line 30 whose outlet opening is placed at thelevel of the lower part of the caisson 8 allows the pressure to be keptequal between the interior of the caisson 8 and the surrounding medium.After emptying the tank 9 of liquid CO filling of the tank with seawateris effected by a pyrotechnic valve 31 and the drainage of air iseffected by way of a pyrotechnic valve 32 mounted on a pipe whichterminates in the tank 9 at a float valve 33.

The method of using the reservoir illustrated in the drawing takes placein the following manner:

The reservoir is lowered in stages in the direction of the under-watersite on which it is to be placed. When the pressure in the interior ofthe tank 9 has been reduced to 18 bars, the electric supply cable isdisconnected from the refrigerator unit 12. 7

At each stage of descent of the reservoir, a suitable quantity of liquidCO is injected into the brinecontained in the enclosures 2 using theelectric valves 18, so that the pressure inside the reservoir is reducedto only slightly higher than that in the surrounding medium in which thereservoir in submerged.

When the reservoir has reached the bottom of the water, the tank 9 isfilled with seawater after it has been emptied of CO and the caisson 8of the reservoir 1 is released by using the explosive cutters 22 and bydetonating the bolts 6. The caisson 8 rises, the purge device 30ensuring during this ascent that the internal and external pressuresacting on the walls of the caisson 8 are kept equal.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as foland connected to the firstenclosure by way of a C0 supply pipe, which is connected to the firstenclosure at a point below the maximum level of the auxiliary solution.

2. A reservoir according to claim 1, wherein the liquid CO chamber isremovably attached to the reservoir body by means of explodable bolts,the supply pipe is associated with an explosive cutter device and the COchamber is provided with a variable buoyancy flotation device.

3. A reservoir according to claim 2, wherein a control valve in the COsupply pipe, the flotation device and the explodable bolts and cutterdevices are associated with electromagnetic control members connected bya cable to a single control station located above the level of the sea.

4. A method of immersing a buoyant reservoir into a body of liquid andcontrolling the pressure in the interior of the reservoir, whichcomprises introducing an aqueous solution into a first enclosure withinsaid reservoir, in amount sufficient to counteract the initial buoyancythereof, introducing a liquefied gas into an auxiliary enclosuredetachably attached to said reservoir, lowering the reservoir in stagesinto the body of liquid, injecting said liquefied gas in stages intosaid first enclosure, said reservoir containing a third enclosurecontaining a flotation gas, said third enclosure being in communicationwith said first enclosure, said liquefied gas being soluble in saidaqueous solution, the injection of said liquefied gas into said aqueoussolution being regulated to maintain the pressure within the reservoirslightly higher than the surrounding medium, and when the reservoir hasreached the bed of said body of liquid, removing said auxiliaryenclosure from the reservoir.

5. A method according to claim 4, wherein the auxiliary solutioncomprises seawater or brine.

6. A method according to claim 4, wherein the compensatory gas is C0 7.A method according to claim 6, wherein the CO is stored at a temperatureof about 20C and under pressure of about 20 bars.

1. A concrete reservoir of large dimensions for immersion in the sea andlocation on the seabed, comprising at least one enclosure distributeduniformly around the periphery of the concrete body of the reservoir,for receiving an auxiliary aqueous solution, at least one secondaryenclosure located centrally of the concrete body of the reservoir, forreceiving a flotation gas, the second enclosure being in constantcommunication with the first enclosure, and a chamber containing liquidCO2 attached to the concrete body of the reservoir and connected to thefirst enclosure by way of a CO2 supply pipe, which is connected to thefirst enclosure at a point below the maximum level of the auxiliarysolution.
 2. A reservoir according to claim 1, wherein the liquid CO2chamber is removably attached to the reservoir body by means ofexplodable bolts, the supply pipe is associated with an explosive cutterdevice and the CO2 chamber is provided with a variable buoyancyflotation device.
 3. A reservoir according to claim 2, wherein a controlvalve in the CO2 supply pipe, the flotation device and the explodablebolts and cutter devices are associated with electromagnetic controlmembers connected by a cable to a single control station located abovethe level of the sea.
 4. A method of immersing a buoyant reservoir intoa body of liquid and controlling the pressure in the interior of thereservoir, which comprises introducing an aqueous solution into a firstenclosure within said reservoir, in amount sufficient to counteract theinitial buoyancy thereof, introducing a liquefied gas into an auxiliaryenclosure detachably attached to said reservoir, lowering the reservoirin stages into the body of liquid, injecting said liquefied gas instages into said first enclosure, said reservoir containing a thirdenclosure containing a flotation gas, said third enclosure being incommunication with said first enclosure, said liquefied gas beingsoluble in said aqueous solution, the injection of said liquefied gasinto said aqueous Solution being regulated to maintain the pressurewithin the reservoir slightly higher than the surrounding medium, andwhen the reservoir has reached the bed of said body of liquid, removingsaid auxiliary enclosure from the reservoir.
 5. A method according toclaim 4, wherein the auxiliary solution comprises seawater or brine. 6.A method according to claim 4, wherein the compensatory gas is CO2.
 7. Amethod according to claim 6, wherein the CO2 is stored at a temperatureof about -20*C and under pressure of about 20 bars.